Gas turbines, as are described for example in printed publication DE-A1-196 19 438, in the turbine section have a rotor which is provided with rotor blade rows and is concentrically enclosed at a distance by a casing. Rings are formed on the casing and carry stator blades which, in common with the rotor blades on the rotor, extend into the hot gas passage which is formed between rotor and casing. Stator blade rows and rotor blade rows alternate in the axial direction or in the direction of the hot gas flow. Heat shield segments, which the rotor blades move past by their blade tips, and which are supplied with cooling air or another cooling medium from an annular cavity which encompasses the heat shield segments, are arranged in a circumferentially distributed manner between adjacent stator blade rows towards the outer limit of the hot gas passage. For cooling, an impingement cooling method, for example, is used, in which the cooling medium, through repeatedly applied openings in an impingement cooling plate, impinges upon the inner side of the wall, which delimits the hot gas passage, of the heat shield segment.
The heat shield segments (“heat shields”) behind the front-stage stator blades of the turbine are exposed to high heat-flow loads. In the region where the rotor blades rotate past, high heat-flow loads occur. High heat-flow loads also occur in the region of the stator blade wake. Wake pressure waves, which are associated with the wake, reduce the pressure margin (back flow margin BFM), i.e. the available pressure difference between hot gas passage and annular cavity, with regard to a hot-gas intrusion.
A “failsafe design” with regard to rubbing (rubbing cracks), loss of sealing (inter heat shield feather seals), part load, ambient conditions (off-ISO design), damage as a result of impact (FOD, i.e. foreign-object damage) and manufacturing tolerances, require an appreciable margin regarding BFM, which at ISO full-load conditions has a negative effect upon the performance.
The number of stator blades in the ring, in the case of conventional solutions, is independent of the number of associated heat shield segments. The number of parts is minimized as far as possible. Since the thermal and mechanical loads of the stator blades are higher, a larger number of stator blades are required in comparison to the number of heat shield segments.